The invention relates to a color electrophotographic copying machine of such a system that color toner images are transferred and superposed one upon another onto a sheet while the sheet is held by a transfer drum. More particularly, the invention is directed to a device that can set a biasing force using a spring with a bias roller oscillated to a biasing position by a cam member, the bias roller being disposed at such a position as to allow the sheet to be held on the transfer drum.
In image forming apparatuses including color electrophotographic copying machines, it is known to dispose developing units for a plurality of colors along a linearly arranged sheet forward paths as disclosed in Japanese Patent Unexamined Publication No. 22227/1984 and the like. This conventional example employs a system in which a plurality of color toner images are transferred onto a sheet from the respective developing units while sequentially superposed one upon another, the sheet passing through the sheet forward paths. Different from such conventional example, e.g., a color electrophotographic copying machine disclosed in Japanese Patent Unexamined Publication No. 107280/1989 is characterized as arranging a plurality of developing units around a cylindrical sheet supporting drum so that a plurality of color toner images can be transferred onto a sheet from the respective developing units and superposed one upon another while the sheet being held on the drum is moving.
Instead of the system in which a plurality of developing units are arranged so that color toner images are transferred onto a sheet from the respective developing units as described above, e.g., an apparatus having the following feature is proposed in Japanese Patent Unexamined Publication No. 238480/1990. That is, a plurality of developing units are arranged around a photoreceptor drum, and a transfer drum supporting a sheet is disposed so as to correspond to the photoreceptor drum. A color electrophotographic copying machine using the above-mentioned transfer drum, which supports a sheet in such a manner as to allow the sheet to adhere to the transfer drum electrostatically, transfers a color toner image onto the sheet so as to be superposed on the sheet every time a color toner image is formed on the photoreceptor drum, separates the sheet from the transfer drum after all the color toner images have been transferred onto the sheet, and forwards the image transferred sheet toward a fusing unit.
As described above, the color electrophotographic copying machine of such system that color toner images formed on the photoreceptor drum are transferred onto a sheet while superposed one upon another with the sheet being supported by the transfer drum employs a transfer drum such as shown in FIG. 3. The transfer drum shown 2 in FIG. 3 disposes a photoreceptor drum 1 so as to confront thereto that supports a sheet. Around the photoreceptor drum 1 a plurality of developing units are disposed, which contain toners of different colors, respectively. One of the developing units forms a toner image of a single color, and the formed color toner image is transferred onto a sheet. A charging means, an image transfer means, a cleaning unit, and the like may be arranged for the photoreceptor drum 1, such as conventional electrophotographic copying machines.
A sheet carrying surface of the transfer drum 2 located at a position corresponding to the photoreceptor drum 1 is formed of a thin plastic film member so that a sheet adheres to such film member electrostatically. In the example shown in FIG. 3, to cause the sheet to adhere to and be held on the surface of the transfer drum 2, an adsorbing corotron 4 is arranged. By controlling discharge from the corotron 4 with a regulating plate 4a, the film that carries the sheet is electrically charged. Then, by pressing the sheet P with a pressure roller 3, the sheet P is subjected to adhere to the surface of the film. At a circumference of the transfer drum 2 and on the inner surface thereof, a transfer corotron 5 is positioned corresponding to the photoreceptor drum 1, so that a color toner image is transferred onto the sheet by the discharge from the transfer corotron 5.
Further, discharge corotrons 6 and a separating corotron 7 are positioned on the upstream side of a claw member for separating the sheet, which serve as auxiliary means for separating the sheet from the surface of the transfer drum. In addition to the above-mentioned components, a cleaning unit 8 is disposed so as to near and leave the transfer drum, so that the surface of the film can be cleaned with the sheet not carried on the surface of the transfer drum. On the upstream side of the sheet adhering position on the surface of the transfer drum are drum discharge corotrons 9, which electrically discharge the film member of the transfer drum before the surface is ready for the adhesion of a next sheet. The pressure roller 3, supported by a not shown oscillating member, performs an auxiliary sheet holding operation by pressing the sheet onto the surface of the transfer drum 2 when the sheet is forwarded by a resist roller unit 13 for adhesion to the surface of the transfer drum.
As described above, if the mechanism for pressing the pressure roller using the support arm member is employed in order to supply the sheet toward the transfer drum, the force for pressing the sheet by the pressure roller can be set to a predetermined value, which is an advantage. At the time, the dransfer drum 2 is constructed by flange frames positioned at both sides of the drum, a plurality of beam frames coupling the both flange frames as the drum and plastic film pasted on the frame of the drum. Accordingly, the plastic film has a portion held by the frame and a portion between the holding frames. In the case such that the sheet holding section of the transfer drum is made use of a film, the resiliency of the film varies from one portion to another, e.g., from the portion having the frame to a portion between the frames, thereby making inconsistent pressure in some cases toward the sheet by the pressure roller. Further, if the pressure force of the pressure roller is set stronger, a large pressure force is applied to sheet over the whole body of the transfer drum, thereby imposing the problem that the operation of supporting the sheet on the transfer drum may not be performed satisfactorily.
Still further, if it is designed to press the pressure roller with a stronger force, then it becomes difficult to remove a sheet when the sheet is jammed at the sheet pressing section or like inconveniences happen because there is no play in the arm member and the like which hold the pressure roller. Still further, since the arm member that supports the pressure roller is driven by a solenoid or the like, when the sheet is jammed between the transfer drum and the pressure roller, it is required that the drive of the solenoid be interrupted and that the pressure roller be distanced from the transfer drum thereafter, which is a cumbersome operation.